Posted
by
kdawson
on Thursday October 25, 2007 @08:51AM
from the portents-in-the-heavens dept.

swordgeek writes "Comet 17P/Holmes, a relatively obscure and (until a few days ago) dim object, has suddenly flared to be literally a million times brighter, going from magnitude 18 to 2.8. It is just outside of the constellation Perseus, which puts it high in the sky and ideal for viewing at this time of year. The comet still appears starlike even in binoculars but should grow to several arcminutes across over the next few nights. The comet is now readily visible to the naked eye. This is a completely unexpected once-in-a-lifetime event, so get out your finest optics (even if it's just your eyes) and go comet watching!"

What about in the UK? Where would I need to look to see this thing?
I know/. is American centered but the world is a pretty big place and those of us not in that continent might like to know where we can/can't see things.

It doesn't matter, it will be in the same place in the sky no matter where on Earth you are.

Well, the only star it's flying near is the big yellow one you see in the daytime. I haven't seen anything about the reason it got brighter, but my guess would be it has an icy core that has been gradually heating internally as it orbited closer to the sun, until suddenly it burst out as steam. If that's the case, maybe it'll get brighter as it gets closer and warmer... or maybe the lid's been taken off the pot and after a day of spewing a ton of stored-up H2O now it'll settle down and get dimmer.

Fortunately for you, Earth has this weird movement called rotation that guarantees the anybody at the same latitude will be able to watch the same phenomenon as long as it is independent of Earth movement and lasts at least a rotation;-)

This is not an eclipse, so you should be able to watch it from the UK, clouds permitting. If you are worried about latitude, you can check the low-cost flights to Spain and come to watch it from here over the weekend. I think you can see Perseus from the UK, anyway.

Magnitude is a log scale, with brighter objects having lower magnitudes. Like so much in astronomy, the reason is historical: when the first guy (Greek, probably) decided to categorize stars by brightness he said the brightest stars should be first magnitude, the next brightest second, and so on. Because of the nature of the human eye, the scale is logarithmic. Objects brighter than what this guy considered first magnitude thus have a magnitude of less than one, or even less than zero.

The numbers are funny (rather than -1, 0, 1, 2) because they're giving you sample objects so you can get an idea about the range of the scale.

This comet orbits between 2.2 and 5.2 AU and it's last closest approach to the Sun was in May, 2007: http://en.wikipedia.org/wiki/17P/Holmes [wikipedia.org]. An AU is the average distance between the Sun and the Earth.

Comet crusts, the dark stuff that is left over after the ice sublimates, are thought to conduct heat slowly. One theory on why we see outbursts as comets move away from the Sun, as this one is doing, is that the warming pulse from the closest approach takes time to sink down to a reservior of carbon monoxide gas which then sublimate internally and blows off fairly large chunks of the comet. Another theory is that the same thermal pulse reaches a reservior of amorphous water ice, which is more common in space than crystalline ice and thus might be present in comets since their formation. When amorphous ice is warmed, it will become crystalline and release energy because the ordered state of crystaline ice is a lower energy state. This can lead to a chain reaction of further crystallization and energy release that could lead to enough warming to cause sublimation in the interior and then do the same kind of thing as in the carbon monoxide scenario. --Get your power from the Sun for what you already pay now: http://mdsolar.blogspot.com/2007/01/slashdot-users-selling-solar.html [blogspot.com]

Just a few facts to ruin the joke.-If a comet is heading directly for us, don't worry. If it is heading where we will be by the time it gets there, on the other hand...-The comet tail is almost not related to its trajectory, but mostly to the direction of the solar wind hitting it, you can approximately draw a line Sun->comet->tail.

So odds are that if we ever have to collide with a comet, we will most likely be idealy placed to see its tail just before the collision.

To expand on this, in the night sky, Perseus looks like a rough square with four curvy lines jutting out of it, and with the northward side of the square "bent" out by a bright star. That star is Mirfak. Perseus isn't as obvious as Cassiopeia, or even Andromeda (just south of Cassiopeia), so if you're in a city and are unfamiliar with the night sky, you might have trouble locating it (just a couple miles out of town should be enough to get a clear enough view). To help orient yourself, the "feet" of Perseus, facing south, look like two triangles (a third triangle, southeast of Perseus and southwest of Andromeda, is the constellation Triangulum).

This comet isn't exactly one to get hyped about. It's not that bright. Magnitude 3 is about as bright as the Little Dipper. You won't see it from the city and you have to know where to look to identify it from better conditions.

Of course, naked eye comets always get a brief mention in the news, even when dim, but this one caught attention because of the dramatic increase in brightness. It's all the more surprising when you consider that this is a short period comet in a relatively circular orbit. It makes it's close approach to the sun frequently, so it doesn't tend to brighten much as it makes the approach, and it has no tail. Even more remarkable, it's currently moving away from the sun, so it would normally be expected to dim, not brighten. Why? Well, it may have had an unusual outgassing event or have impacted another object. Beyond that, I don't have any good guesses.

The brightest comet in decades was McNaught, which made a show last winter. Unfortunately, it was very close to the sun, so it rose barely after sunrise and set barely after sunset and was therefore hard to observe. However, it quickly got brighter than Venus and eventually was so bright (M -6) that a clever observer in clear, dry air could spot it during the day, a scant few degrees from the sun.

It was a little more friendly to observers in the southern hemisphere, and its huge, striated tail was spectacular. Here's a picture [nasa.gov].

Kohoutek wasn't all that bright. Probably the best observer's comet last century was Hale-Bopp, which was very photogenic and had a remarkable double tail. I wasn't alive for Halley, which has a lot of historical significance, but it's latest pass wasn't very impressive.

I'm slightly appalled.I write up what I consider to be an interesting story for/. readers, check my grammar and links, and then click submit. Lo and behold, it gets accepted. Cool!

Then I read what I supposedly wrote, and find that words have been put in my mouth. Specifically:

"The comet still appears starlike even in binoculars but should grow to several arcminutes across over the next few nights. "

I did NOT write that. I didn't suggest it. The comet does NOT appear starlike in binoculars at all--it's quite a clear extended disk. ONE person quoted in Sky & Telescope believes that it will expand to several arcminutes in size, but that's strictly conjecture.

Furthermore, I didn't use the word "millionfold" in my subject line. That was kdawson again./. editors: If you're going to actually edit submissions (and why start now, after ignoring fundamental grammatical errors and duplicate stories for a decade?) then get it right!

Just to be clear on something, I never used the word millionfold in my submission. That was kdawson editing my post for his own glory.

Incidentally, the term has been used both ways, and has etymologically distinct roots, so millionfold meaning 'a million times' is valid.I can't link directly to it, at the sixth entry (-fold) at Miriam Webster's [m-w.com].

Nonetheless, it wasn't my word. Neither was the claim that the comet is 'starlike in binoculars.'

Nope. This comet made its closest approach to the sun back in May. It's now moving away from the sun. It also follows a relatively circular orbit compared to most bright comets, so significant changes in brightness are unexpected.

However, there may have been an unusually large pocket of vapor that form some reason burst out at this point (out-gassing), or it might have been hit by a smaller object.